1. Field of the Invention
The present invention relates to a motion vector detection technique, e.g., a device having a function for detecting an interframe motion vector for coding motion images with an image coding method including an interframe prediction mode.
2. Description of the Related Art
The rapid development of the broadband network has increased consumer expectations for the service that provide high-quality motion images. On the other hand, large capacity storage media such as DVD and so forth are used for storing high-quality motion images. This increases the segment of the users who enjoy high-quality images. A compression coding method is an indispensable technique for transmission of motion images via a communication line, and storing the motion images in a storage medium. Examples of international standards of motion image compression coding techniques include the MPEG-4 standard, and the H.264/AVC standard. Furthermore, SVC (Scalable Video Coding) is known, which is a next-generation image compression technique that includes both high image quality stream and low image quality stream functions.
With compression coding and decoding of motion images, the motion images are stored in frame memory in increments of frames, and motion compensation is performed with reference to the frame memory. This requires high frequency data transmission from the frame memory. In particular, creation of higher quality motion images requires motion detection in increments of blocks each of which is formed of a small number of image pixels. This increases the data amount used for the motion compensation. Accordingly, the demand for memory bandwidth can easily lead to a bottleneck in the processing. The Japanese Patent Application Laid-open No. 11-298903 discloses a digital image decoding device having a function of improving the bandwidth usage efficiency of frame memory.
On the other hand, the Japanese Patent Application Laid-open No. 11-262015 discloses a hierarchical motion vector detection method as follows. First, a coding target image is reduced to a reduced image at a reduced resolution. Then, an approximate motion vector is detected based upon the reduced image thus obtained. Subsequently, the motion vector is detected based upon the original image at the original resolution with reference to the approximate motion vector.
At the time of compression coding of motion images, motion detection is performed for the target macro block of a coding target frame. With the motion detection, the macro block matching the target macro block is detected within the pixel region corresponding to a predetermined search region in a reference frame while reading out the pixel region from the frame memory. With such motion detection, the macro block matching the target macro block is detected within the reference frame by repeated detection. This requires a great number of readouts from the frame memory, increasing the data transmission amount. Such data transmission takes up most of the transmission bandwidth of the frame memory. As a result, access to the frame memory becomes bottlenecked, leading to a problem of reduced processing speed for the compression coding.
On the other hand, with the aforementioned hierarchical motion vector detection method, a portion of the reference image corresponding to the detection region is read out from the frame memory, and a reduced image is created based upon this partial image thus read out, for each coding target macro block. This requires extremely high frequency data exchange via a memory bus, leading to a bottlenecking problem, i.e., a problem of limited processing speed.